Domain (biology)

Life has three big groups.

These groups help us sort all living things. One group is for tiny life. It has no center in its cells.

One group is also very tiny. These are different from the first group. The last group is for plants and animals. These have a center in their cells. This group also includes fungi. This system does not include viruses. Viruses are not part of these three groups. It is a way to see how life is linked.

Scientists sort all life into big groups. These groups are called domains.

In 1990, three scientists named a new system. It has three domains. These are Archaea, Bacteria, and Eukarya.

Archaea and Bacteria are tiny. They are prokaryotes. This means their cells have no nucleus. A nucleus is a center that holds genetic material. Archaea are special. They can live in very hot or salty places. This is because of how their cell parts are made. Bacteria are also tiny. But they have different cell parts than Archaea.

The third group is Eukarya. We call these living things eukaryotes. Their cells have a nucleus. This group includes plants, animals, and fungi. Some scientists think Eukarya came from Archaea.

This system does not include viruses. Viruses are not made of cells. Other people have tried new ways to sort life. One idea uses five domains. That system includes viruses and prions. Prions are also not made of cells. But the three-domain system is very common.

Scientists use big groups to sort all living things. These groups are called domains. A domain is the highest rank in the tree of life.

It is the biggest way to organize every organism. This system helps us see how life is related. It is a way to make sense of the whole world. Without these groups, life would be very hard to study.

How does this work? Scientists look at the tiny parts inside a cell. They check the RNA, which is a special marker in the cell.

They also look at the cell membrane. This is the outer layer of a cell. In the three-domain system, life is split into three branches. These are Archaea, Bacteria, and Eukarya. Archaea and Bacteria are prokaryotes. These are tiny living things with no nucleus. Eukarya are eukaryotes. These have a nucleus to hold their genetic material.

This way of sorting life is not very old. Carl Linnaeus made a famous system in the mid-1700s. Later, Charles Darwin helped improve how we group things. But it was hard to group bacteria back then. In 1977, Carl Woese found a big clue. He compared the RNA sequences of cells. He found three branches instead of two. Along with Otto Kandler and Mark Wheelis, he shared this in 1990.

Each domain has its own special facts. Archaea can live in very hot or salty places. This is because of their unique cell membranes. Some are as small as 0.1 micrometers. Bacteria are also tiny, but they are different from Archaea. They have different RNA and different cell membranes. Eukarya is a huge group. It includes four kingdoms: Plantae, Protista, Animalia, and Fungi.

These are the plants, animals, and fungi we see every day.

This system is like a giant map of life. It connects the tiny things we cannot see to the big things we can. You are part of the Eukarya domain!

Most of this system does not include viruses. Viruses are not made of cells. Some scientists, like Stefan Luketa, suggest new systems. He proposed five domains in 2012. His system includes viruses and prions. These are things that are not made of cells. Even so, the three-domain system is very common today.

In biological taxonomy, a domain is the highest possible rank for all living organisms. It is the broadest way to categorize life on Earth. Scientists use domains to organize every living thing into massive groups. This system helps us understand the relationships between all life forms.

By using domains, researchers can see how different species are connected through history. This classification is essential for studying the entire tree of life.

To build this system, scientists look at the tiny molecular structures inside a cell. One of the most important tools is the study of ribosomal RNA. This is a specific type of RNA found in ribosomes, which are parts of the cell. Each domain has its own unique ribosomal RNA sequences. Scientists also examine the cell membrane, the outer layer of the cell. The biochemistry of these membranes differs greatly between the different domains. These molecular markers act like biological fingerprints that reveal how organisms are related.

The three-domain system divides life into Archaea, Bacteria, and Eukarya. The first two, Archaea and Bacteria, are classified as prokaryotes. Prokaryotes are single-celled microorganisms that lack a membrane-bound nucleus. This means they do not have a protective envelope around their genetic material. Eukarya is the third domain, consisting of eukaryotes. Eukaryotes are defined by having a cell nucleus and other membrane-bound organelles.

These organelles are specialized structures that perform specific jobs within the cell.

Classification history has changed as our tools have improved. In the mid-eighteenth century, Carl Linnaeus created a famous taxonomy system. Later, Charles Darwin provided more information to improve these groupings. However, early scientists struggled to classify bacteria because they had few observable features. In 1974, Royall T. Moore proposed using the term "dominion," but it was not widely used. A major breakthrough occurred in 1977 when Carl Woese compared nucleotide sequences of 16s ribosomal RNA. Along with Otto Kandler and Mark Wheelis, he introduced the three-domain system in 1990.

Each domain has very specific physical characteristics and sizes. Archaea are prokaryotic cells with unique membrane lipids. These lipids consist of branched hydrocarbon chains attached to glycerol by ether linkages. These chemical bonds help Archaea survive in extreme heat or high acidity. Some examples include halophiles, which love salt, and hyperthermophiles, which love heat. Archaea range from 0.1 μm to 15 μm in diameter. They can grow up to 200 μm in length. The genus Thermoplasma contains some of the smallest known Archaea.

Bacteria are also prokaryotes, but they are chemically distinct from Archaea. Bacteria use phospholipid bilayers for their cell membranes instead of ether linkages. They also possess different internal RNA structures in their ribosomes. Bacteria show an incredible amount of diversity. This diversity is complicated by the exchange of genes between different lineages. This gene exchange can make it hard to organize bacteria into a perfect tree. Instead, some scientists suggest their relationships look more like a network. Examples of bacteria include cyanobacteria and mycoplasmas.

The domain Eukarya is massive and includes four main kingdoms. These kingdoms are Plantae, Protista, Animalia, and Fungi. Eukaryotes are much more complex than the other two domains. Some scientific theories, like the eocyte hypothesis, suggest Eukarya evolved from Archaea. This hypothesis proposes that Eukarya is actually a branch within the Archaea domain. This would result in a two-domain system of Bacteria and Archaea.

It is important to note what is left out of this system. Most domain models do not include non-cellular life, such as viruses. Because viruses do not have cells, they do not fit the standard definition. In 2012, Stefan Luketa proposed a five-dominion system to include them. His system adds Virusobiota, which contains viruses, and Prionobiota, which contains prions. Prions are acellular and do not have nucleic acids. Other scientists have also suggested different ways to group life, such as the two-empire system. These debates show that our understanding of life is always evolving.